The Attribute Controller
Registers are accessed via a pair of registers, the Attribute Address/Data
Register and the Attribute Data Read Register. See the Accessing
the VGA Registers section for more detals. The Address/Data Register
is located at port 3C0h and the Data Read Register is located at port 3C1h.

Attribute Address Register(3C0h)

7

6

5

4

3

2

1

0

PAS

Attribute Address

PAS -- Palette Address Source"This bit is set to 0 to load color values to the registers in the
internal palette. It is set to 1 for normal operation of the attribute
controller. Note: Do not access the internal palette while this bit is
set to 1. While this bit is 1, the Type 1 video subsystem disables accesses
to the palette; however, the Type 2 does not, and the actual color value
addressed cannot be ensured."

Attribute AddressThis field specifies the index value of the attribute register to be
read or written.

Palette Registers (Index 00-0Fh)

7

6

5

4

3

2

1

0

Internal Palette Index

Internal Palette Index"These 6-bit registers allow a dynamic mapping between the text
attribute or graphic color input value and the display color on the CRT
screen. When set to 1, this bit selects the appropriate color. The Internal
Palette registers should be modified only during the vertical retrace interval
to avoid problems with the displayed image. These internal palette values
are sent off-chip to the video DAC, where they serve as addresses into
the DAC registers."

Attribute Mode Control Register
(Index 10h)

7

6

5

4

3

2

1

0

P54S

8BIT

PPM

BLINK

LGE

MONO

ATGE

P54S -- Palette Bits 5-4 Select"This bit selects the source for the P5 and P4 video bits that act
as inputs to the video DAC. When this bit is set to 0, P5 and P4 are the
outputs of the Internal Palette registers. When this bit is set to 1, P5
and P4 are bits 1 and 0 of the Color Select register."
8BIT -- 8-bit Color Enable"When this bit is set to 1, the video data is sampled so that eight
bits are available to select a color in the 256-color mode (0x13). This
bit is set to 0 in all other modes."

PPM -- Pixel Panning Mode

This field allows the upper half of the screen to pan independently
of the lower screen. If this field is set to 0 then nothing special occurs
during a successful line compare (see the Line
Compare field.) If this field is set to 1, then upon a successful line
compare, the bottom portion of the screen is displayed as if the Pixel
Shift Count and Byte Panning fields are
set to 0.
BLINK - Blink Enable"When this bit is set to 0, the most-significant bit of the attribute
selects the background intensity (allows 16 colors for background). When
set to 1, this bit enables blinking."

LGA - Line Graphics Enable

This field is used in 9 bit wide character modes to provide continuity
for the horizontal line characters in the range C0h-DFh. If this field
is set to 0, then the 9th column of these characters is replicated from
the 8th column of the character. Otherwise, if it is set to 1 then the
9th column is set to the background like the rest of the characters.

MONO - Monochrome Emulation

This field is used to store your favorite bit. According to IBM, "When
this bit is set to 1, monochrome emulation mode is selected. When this
bit is set to 0, color |emulation mode is selected." It is present and
programmable in all of the hardware but it apparently does nothing. The
internal palette is used to provide monochrome emulation instead.

Overscan Palette Index"These bits select the border color used in the 80-column alphanumeric
modes and in the graphics modes other than modes 4, 5, and D. (Selects
a color from one of the DAC registers.)"

Pixel Shift Count"These bits select the number of pels that the video data is shifted
to the left. PEL panning is available in both alphanumeric and graphics
modes."

Color Select Register (Index 14h)

7

6

5

4

3

2

1

0

Color Select 7-6

Color Select 5-4

Color Select 7-6"In modes other than mode 13 hex, these are the two most-significant
bits of the 8-bit digital color value to the video DAC. In mode 13 hex,
the 8-bit attribute is the digital color value to the video DAC. These
bits are used to rapidly switch between sets of colors in the video DAC."
Color Select 5-4"These bits can be used in place of the P4 and P5 bits from the
Internal Palette registers to form the 8-bit digital color value
to the video DAC. Selecting these bits is done in the Attribute Mode Control
register (index 0x10). These bits are used to rapidly switch between colors
sets within the video DAC."